1. Field of the Invention
This invention relates to method and apparatus for controlling back pressure of an injection plunger in the step of charging material in an injection molding machine using an electric motor as a driving source.
2. Description of the Prior Art
In an injection molding machine of this type, rotation and forward movement of an injection plunger 2 houses within an injection heating tube 1 are effected by means of an electric motor 3, as illustrated in FIG. 6.
The turning force of the motor 3 is transmitted to the injection plunger 2 through gears 7 and 8 provided on a transmission shaft 4 along with clutches 5 and 6.
The gear 7 is meshed with a gear 9 on a rotational shaft 2a at the rear of the plunger, and the turning force caused by the motor 3 is transmitted to the gear 9 so that the injection plunger 2 may be rotated.
The gear 8 is meshed with a gear 12 provided on a screw shaft 11 of a plunger moving member 10, and the turning force caused by the motor 3 is transmitted from the gear 12 to the screw shaft 11 to act as a thrust of the axially movable plunger moving member 10 connected to the rotational shaft 2a, thereby forwardly moving the injection plunger 2.
The back pressure in the step of charging material is permitted to be controlled by means of an electromagnetic brake 13 connected to the rear end of the screw shaft 11. The electromagnetic brake 13 is secured to the rear wall of a housing 14 with the injection heating tube 1 mounted on the front wall thereof.
In the molding machine constructed as described above, rotation of the gear 9 causes the injection plunger 2 to rotate along with the rotational shaft 2a whereby material from a hopper 16 may be transferred forwardly of the plunger.
The backing force generated in the injection plunger 2 upon transfer of material is caused to generate the turning force in the backing direction of the plunger in the screw shaft 11 through the plunger moving member 10. At the same time, the control is made by the electromagnetic brake 13 to control the torque of the screw shaft 11, thereby charging the material while applying the plunger back pressure to a molten material.
The technique disclosed in Japanese Patent Application Laid-Open No. 58-179631 as a method for controlling back pressure of a plunger in the injection molding machine as mentioned above has an open loop control system.
There the plunger back pressure is controlled by an electrically-driven brake, a screw shaft, a member for converting the turning force of the screw shaft into thrust of the injection plunger and the like are interposed between the injection plunger and the back pressure control means. Therefore, pressure received by the back pressure control means from the injection plunger is indirect, thus posing a problem of inaccurate control because of a frictional force between the aforesaid interposed members.
To overcome the problem, the present inventors have previously developed (see Japanes Patent Application No. 59-153340) a method for controlling back pressure of a plunger, in which an electric motor is used as a back presure control device, and feed-back control and the turning force of the motor are utilized.
In this prior art, as shown in FIG. 7, an electric motor 17 is provided in place of the aforesaid electromagnetic brake on the housing 14, and a pressure sensor in the form of a strain gauge is mounted as a back pressure sensor 15 at a part subjected to reaction of the injection plunger of the rear wall of the housing, for example, at the wall of a portion where the rear end of the screw shaft 11 is held. It is to be noted that the back pressure sensor 15 can be replaced by a resin pressure sensor generally used, in which case it is provided on the injection heating tube 1 for measuring molten resin pressure.
A pressure detection signal outputted from the back pressure sensor 15 is converted by a converter circuit 20 (FIG. 8) into a pressure detection signal ep which is formed of a voltage signal and applied to an adding point A. A pressure command signal epi having a voltage level which is controlled according to a value set by a back pressure setting device 21 is applied from the device 21 to the adding point A. As the result, a differential signal of epi-ep=.DELTA.ep is outputted from the adding point A, amplified by a pressure control amplifier 22 and applied to an adding point B as a current command signal .DELTA.epo. A current of the motor 17 for deciding the output torque of the motor 17 is detected by a current detector 23, and the curent detection signal is converted by a converter circuit 24 into a current detection signal ei formed of a voltage signal and applied to the adding point B. As a consequence, a differential signal of .DELTA.-epo-ei=.DELTA.ei is outputted from the adding point B, and the differential signal .DELTA.ei is amplified by a current control amplifier 25 into .DELTA.eio, after which it is applied to a power converter 26.
The power converter 26 is composed of an ignition control circuit using a thyristor or a pluse width control circuit using a transistor, whereby a predetermined current is applied to the motor according to the signal .DELTA.eio to drive the motor 17.
However, the back pressure control apparatus as described above has the problem as noted below because the maximum output current (the maximum output torque of the motor) has a given value determined according to the specification of the motor 17 and the power converter 26 irrespective of the pressure command value.
As shown in FIG. 9, assuming that the material charging step after completion of injection step begins at time T.sub.o at which the initial value of the back pressure of the injection plunger is in a state of 0 to output the pressure command signal epi having a stepwise voltage waveform from the back pressure setting device 21, the output of the motor 17 varies to be greater or smaller than the command pressure in the transient period till the back pressure comes into coincidence with the set value thereof. Accordingly, the back pressure detection signal ep changes in the mode of damping oscillation as shown and comes into coincidence with the pressure command signal epi after a lapse of a predetermined time.
In this case, the amplification degree of the pressure control amplifier 22 is the degree required for controlling the back pressure, and if the pressure command value is large, the differential signal .DELTA.ep between the pressure command signal epi and the pressure detection signal ep increases, and the current command signal .DELTA.epo excessively increases as a consequence of which if the differential signal .DELTA.epo is positive, the injection plunger 2 is pressed by the motor 17 with a force greater than needed (3 to 5 times of the rated value of the back pressure), whereas if the differential signal .DELTA.epo is negative, the plunger is forced back by the load of molten material of the injection plunger.
As a result, the injection plunger is repeatedly reciprocated, and the back pressure repeats its great oscillation and gradually reaches the pressure command value.
The over-shoot .beta. of the pressure detection signal ep and the setting time t are determined according to the magnitude of the current command signal .DELTA.epo, and the rising angle .alpha. of the pressure detection signal ep is determined according to the amplification degree of the pressure control amplifier 22. However, in the method in which the maximum output of the motor 17 is determined to be constant irrespective of the pressure command value as in prior art, if the pressure command value is large, the current command signal .DELTA.epo excessively increases, and the setting time till the back pressure reaches the set value of the back pressure is prolonged and the peak value of the back pressure excessively increases, posing a problem.